diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9e741b5..bb5dc6b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -53,7 +53,7 @@ jobs:
       - run: mix compile --warnings-as-errors
         if: ${{ matrix.lint }}
 
-      - run: MIX_ENV=test mix coveralls.github
+      - run: MIX_ENV=test mix coveralls.github --warnings-as-errors
 
       - name: Retrieve PLT Cache
         uses: actions/cache@v1
diff --git a/CHANGELOG.md b/CHANGELOG.md
index e11755f..eea3499 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,11 @@ The generated namespaces are much more flexible now and compile faster.
 - When defining an alias for a term of vocabulary which would be invalid as an
   Elixir term, the original term is now implicitly ignored and won't any longer
   be returned by the `__terms__/0` function of a `RDF.Vocabulary.Namespace`.
+- `RDF.Graph.build/2` blocks are now wrapped in a function, so the aliases and
+  import no longer affect the caller context. `alias`es in the caller context are 
+  still available in the build block, but `import`s not and must be reimported in 
+  the build block. Variables in the caller context are also no longer available 
+  build block.
 - `RDF.Data.merge/2` and `RDF.Data.equal?/2` are now commutative, i.e. structs
   which implement the `RDF.Data` protocol can be given also as the second argument
   (previously custom structs with `RDF.Data` protocol implementations always
diff --git a/lib/rdf.ex b/lib/rdf.ex
index 0cf2fd7..dafc91d 100644
--- a/lib/rdf.ex
+++ b/lib/rdf.ex
@@ -342,5 +342,6 @@ defmodule RDF do
   defdelegate __base_iri__(), to: RDF.NS.RDF
   defdelegate __terms__(), to: RDF.NS.RDF
   defdelegate __iris__(), to: RDF.NS.RDF
+  defdelegate __strict__(), to: RDF.NS.RDF
   defdelegate __resolve_term__(term), to: RDF.NS.RDF
 end
diff --git a/lib/rdf/graph.ex b/lib/rdf/graph.ex
index 7717cf7..0a1c212 100644
--- a/lib/rdf/graph.ex
+++ b/lib/rdf/graph.ex
@@ -145,12 +145,7 @@ defmodule RDF.Graph do
   For a description of the DSL see [this guide](https://rdf-elixir.dev/rdf-ex/description-and-graph-dsl.html).
   """
   defmacro build(opts \\ [], do: block) do
-    Builder.build(
-      block,
-      __CALLER__,
-      Builder.namespace_context_mod(__CALLER__),
-      opts
-    )
+    Builder.build(block, __CALLER__, Builder.builder_mod(__CALLER__), opts)
   end
 
   @doc """
diff --git a/lib/rdf/graph_builder.ex b/lib/rdf/graph_builder.ex
index f4a06db..2180fb3 100644
--- a/lib/rdf/graph_builder.ex
+++ b/lib/rdf/graph_builder.ex
@@ -18,40 +18,54 @@ defmodule RDF.Graph.Builder do
   end
 
   def build(do_block, env, opts) do
-    build(do_block, env, namespace_context_mod(env), opts)
+    build(do_block, env, builder_mod(env), opts)
   end
 
-  def build({:__block__, _, block}, env, namespace_context_mod, opts) do
+  def build({:__block__, _, block}, env, builder_mod, opts) do
     env_aliases = env_aliases(env)
+    block = expand_aliased_modules(block, env_aliases)
+    non_strict_ns = extract_non_strict_ns(block)
     {declarations, data} = Enum.split_with(block, &declaration?/1)
-    {base, declarations} = extract_base(declarations, env_aliases)
+    {base, declarations} = extract_base(declarations)
     base_string = base_string(base)
     data = resolve_relative_iris(data, base_string)
     declarations = resolve_relative_iris(declarations, base_string)
+    {prefixes, ad_hoc_ns, declarations} = extract_prefixes(declarations, builder_mod, env)
+    non_strict_ns = (non_strict_ns ++ ad_hoc_ns) |> Enum.uniq()
 
-    {prefixes, declarations} =
-      extract_prefixes(declarations, env_aliases, namespace_context_mod, env)
+    mod_body =
+      quote do
+        for mod <- unquote(non_strict_ns) do
+          @compile {:no_warn_undefined, mod}
+        end
+
+        def build(opts) do
+          alias RDF.XSD
+          alias RDF.NS.{RDFS, OWL}
+
+          import RDF.Sigils
+          import Helper
+
+          unquote(declarations)
+
+          RDF.Graph.Builder.do_build(
+            unquote(data),
+            opts,
+            unquote(prefixes),
+            unquote(base_string)
+          )
+        end
+      end
+
+    Module.create(builder_mod, mod_body, Macro.Env.location(env))
 
     quote do
-      alias RDF.XSD
-      alias RDF.NS.{RDFS, OWL}
-
-      import RDF.Sigils
-      import Helper
-
-      unquote(declarations)
-
-      RDF.Graph.Builder.do_build(
-        unquote(data),
-        unquote(opts),
-        unquote(prefixes),
-        unquote(base_string)
-      )
+      apply(unquote(builder_mod), :build, [unquote(opts)])
     end
   end
 
-  def build(single, env, namespace_context_mod, opts) do
-    build({:__block__, [], List.wrap(single)}, env, namespace_context_mod, opts)
+  def build(single, env, builder_mod, opts) do
+    build({:__block__, [], List.wrap(single)}, env, builder_mod, opts)
   end
 
   @doc false
@@ -60,8 +74,8 @@ defmodule RDF.Graph.Builder do
     |> Graph.add(Enum.filter(data, &rdf?/1))
   end
 
-  def namespace_context_mod(env) do
-    Module.concat(env.module, "GraphBuilderNS#{random_number()}")
+  def builder_mod(env) do
+    Module.concat(env.module, "GraphBuilder#{random_number()}")
   end
 
   defp graph_opts(opts, prefixes, base) do
@@ -107,14 +121,11 @@ defmodule RDF.Graph.Builder do
     end)
   end
 
-  defp extract_base(declarations, env_aliases) do
+  defp extract_base(declarations) do
     {base, declarations} =
       Enum.reduce(declarations, {nil, []}, fn
         {:@, line, [{:base, _, [{:__aliases__, _, ns}] = aliases}]}, {_, declarations} ->
-          {
-            ns |> expand_module(env_aliases) |> Module.concat(),
-            [{:alias, line, aliases} | declarations]
-          }
+          {Module.concat(ns), [{:alias, line, aliases} | declarations]}
 
         {:@, _, [{:base, _, [base]}]}, {_, declarations} ->
           {base, declarations}
@@ -126,49 +137,53 @@ defmodule RDF.Graph.Builder do
     {base, Enum.reverse(declarations)}
   end
 
-  defp extract_prefixes(declarations, env_aliases, namespace_context_mod, env) do
-    {prefixes, declarations} =
-      Enum.reduce(declarations, {[], []}, fn
-        {:@, line, [{:prefix, _, [{:__aliases__, _, ns}] = aliases}]}, {prefixes, declarations} ->
+  defp extract_prefixes(declarations, builder_mod, env) do
+    {prefixes, ad_hoc_ns, declarations} =
+      Enum.reduce(declarations, {[], [], []}, fn
+        {:@, line, [{:prefix, _, [{:__aliases__, _, ns}] = aliases}]},
+        {prefixes, ad_hoc_ns, declarations} ->
           {
-            [prefix(ns, env_aliases) | prefixes],
+            [prefix(ns) | prefixes],
+            ad_hoc_ns,
             [{:alias, line, aliases} | declarations]
           }
 
         {:@, line, [{:prefix, _, [[{prefix, {:__aliases__, _, ns} = aliases}]]}]},
-        {prefixes, declarations} ->
+        {prefixes, ad_hoc_ns, declarations} ->
           {
-            [prefix(prefix, ns, env_aliases) | prefixes],
+            [prefix(prefix, ns) | prefixes],
+            ad_hoc_ns,
             [{:alias, line, [aliases]} | declarations]
           }
 
-        {:@, line, [{:prefix, _, [[{prefix, uri}]]}]}, {prefixes, declarations}
+        {:@, line, [{:prefix, _, [[{prefix, uri}]]}]}, {prefixes, ad_hoc_ns, declarations}
         when is_binary(uri) ->
-          ns = ad_hoc_namespace(prefix, uri, namespace_context_mod, env)
+          ns = ad_hoc_namespace(prefix, uri, builder_mod, env)
 
           {
-            [prefix(prefix, ns, env_aliases) | prefixes],
+            [prefix(prefix, ns) | prefixes],
+            [Module.concat(ns) | ad_hoc_ns],
             [{:alias, line, [{:__aliases__, line, ns}]} | declarations]
           }
 
         {:@, _, [{:prefix, _, _}]} = expr, _ ->
           raise Error, "invalid @prefix expression:\n\t#{Macro.to_string(expr)}"
 
-        declaration, {prefixes, declarations} ->
-          {prefixes, [declaration | declarations]}
+        declaration, {prefixes, ad_hoc_ns, declarations} ->
+          {prefixes, ad_hoc_ns, [declaration | declarations]}
       end)
 
-    {prefixes, Enum.reverse(declarations)}
+    {prefixes, ad_hoc_ns, Enum.reverse(declarations)}
   end
 
-  defp prefix(namespace, env_aliases) do
+  defp prefix(namespace) do
     namespace
     |> determine_prefix()
-    |> prefix(namespace, env_aliases)
+    |> prefix(namespace)
   end
 
-  defp prefix(prefix, namespace, env_aliases) do
-    {prefix, namespace |> expand_module(env_aliases) |> Module.concat()}
+  defp prefix(prefix, namespace) do
+    {prefix, Module.concat(namespace)}
   end
 
   defp determine_prefix(namespace) do
@@ -180,9 +195,9 @@ defmodule RDF.Graph.Builder do
     |> String.to_atom()
   end
 
-  defp ad_hoc_namespace(prefix, uri, namespace_context_mod, env) do
+  defp ad_hoc_namespace(prefix, uri, builder_mod, env) do
     {:module, module, _, _} =
-      namespace_context_mod
+      builder_mod
       |> Module.concat(prefix |> Atom.to_string() |> Macro.camelize())
       |> Vocabulary.Namespace.create!(uri, [], env, strict: false)
 
@@ -211,6 +226,19 @@ defmodule RDF.Graph.Builder do
     raise Error, message: "invalid RDF data: #{inspect(invalid)}"
   end
 
+  defp expand_aliased_modules(ast, env_aliases) do
+    Macro.prewalk(ast, fn
+      {:__aliases__, [alias: false], _} = alias ->
+        alias
+
+      {:__aliases__, _, module} ->
+        {:__aliases__, [alias: false], expand_module(module, env_aliases)}
+
+      other ->
+        other
+    end)
+  end
+
   defp expand_module([first | rest] = module, env_aliases) do
     if full = env_aliases[first] do
       full ++ rest
@@ -233,6 +261,23 @@ defmodule RDF.Graph.Builder do
     String.to_atom(short)
   end
 
+  defp extract_non_strict_ns(block) do
+    modules =
+      block
+      |> Macro.prewalker()
+      |> Enum.reduce([], fn
+        {:__aliases__, _, mod}, modules -> [Module.concat(mod) | modules]
+        _, modules -> modules
+      end)
+      |> Enum.uniq()
+
+    for module <- modules, non_strict_vocab_namespace?(module), do: module
+  end
+
+  defp non_strict_vocab_namespace?(mod) do
+    Vocabulary.Namespace.vocabulary_namespace?(mod) and not mod.__strict__()
+  end
+
   defp random_number do
     :erlang.unique_integer([:positive])
   end
diff --git a/test/unit/graph_builder_test.exs b/test/unit/graph_builder_test.exs
index f175127..a733a13 100644
--- a/test/unit/graph_builder_test.exs
+++ b/test/unit/graph_builder_test.exs
@@ -18,6 +18,7 @@ defmodule RDF.Graph.BuilderTest do
 
   @compile {:no_warn_undefined, __MODULE__.TestNS.EX}
   @compile {:no_warn_undefined, __MODULE__.TestNS.Custom}
+  @compile {:no_warn_undefined, RDF.Test.Case.EX}
 
   alias TestNS.EX
   alias RDF.NS
@@ -317,51 +318,53 @@ defmodule RDF.Graph.BuilderTest do
   end
 
   test "RDF.XSD is aliased" do
-    # we're wrapping this in a function to isolate the alias
     graph =
-      (fn ->
-         RDF.Graph.build do
-           EX.S |> EX.p(XSD.byte(42))
-         end
-       end).()
+      RDF.Graph.build do
+        EX.S |> EX.p(XSD.byte(42))
+      end
 
     assert graph == RDF.graph(EX.S |> EX.p(RDF.XSD.byte(42)))
   end
 
   test "default aliases" do
-    # we're wrapping this in a function to isolate the alias
     graph =
-      (fn ->
-         RDF.Graph.build do
-           OWL.Class |> RDFS.subClassOf(RDFS.Class)
-         end
-       end).()
+      RDF.Graph.build do
+        OWL.Class |> RDFS.subClassOf(RDFS.Class)
+      end
 
     assert graph == RDF.graph(NS.OWL.Class |> NS.RDFS.subClassOf(NS.RDFS.Class))
   end
 
   test "alias" do
-    # we're wrapping this in a function to isolate the alias
     graph =
-      (fn ->
-         RDF.Graph.build do
-           alias TestNS.Custom
-           Custom.S |> Custom.p(Custom.O)
-         end
-       end).()
+      RDF.Graph.build do
+        alias TestNS.Custom
+        Custom.S |> Custom.p(Custom.O)
+      end
 
     assert graph == RDF.graph(TestNS.Custom.S |> TestNS.Custom.p(TestNS.Custom.O))
   end
 
-  test "import" do
-    # we're wrapping this in a function to isolate the import
+  test "aliasing an already taken name" do
     graph =
-      (fn ->
-         RDF.Graph.build do
-           import TestNS.ImportTest
-           EX.S |> foo(TestNS.ImportTest.Bar)
-         end
-       end).()
+      RDF.Graph.build do
+        alias RDF.Test.Case.EX, as: EX2
+        {EX2.S, EX.p(), EX2.foo()}
+      end
+
+    quote do
+      alias RDF.Test.Case.EX, as: EX2
+    end
+
+    assert graph == RDF.graph(RDF.Test.Case.EX.S |> EX.p(RDF.Test.Case.EX.foo()))
+  end
+
+  test "import" do
+    graph =
+      RDF.Graph.build do
+        import TestNS.ImportTest
+        EX.S |> foo(TestNS.ImportTest.Bar)
+      end
 
     assert graph == RDF.graph(EX.S |> TestNS.ImportTest.foo(TestNS.ImportTest.Bar))
   end
@@ -394,15 +397,12 @@ defmodule RDF.Graph.BuilderTest do
 
   describe "@prefix" do
     test "for vocabulary namespace with explicit prefix" do
-      # we're wrapping this in a function to isolate the alias
       graph =
-        (fn ->
-           RDF.Graph.build do
-             @prefix cust: TestNS.Custom
+        RDF.Graph.build do
+          @prefix cust: TestNS.Custom
 
-             Custom.S |> Custom.p(Custom.O)
-           end
-         end).()
+          Custom.S |> Custom.p(Custom.O)
+        end
 
       assert graph ==
                RDF.graph(TestNS.Custom.S |> TestNS.Custom.p(TestNS.Custom.O),
@@ -411,15 +411,12 @@ defmodule RDF.Graph.BuilderTest do
     end
 
     test "for vocabulary namespace with auto-generated prefix" do
-      # we're wrapping this in a function to isolate the alias
       graph =
-        (fn ->
-           RDF.Graph.build do
-             @prefix TestNS.Custom
+        RDF.Graph.build do
+          @prefix TestNS.Custom
 
-             Custom.S |> Custom.p(Custom.O)
-           end
-         end).()
+          Custom.S |> Custom.p(Custom.O)
+        end
 
       assert graph ==
                RDF.graph(TestNS.Custom.S |> TestNS.Custom.p(TestNS.Custom.O),
@@ -428,15 +425,12 @@ defmodule RDF.Graph.BuilderTest do
     end
 
     test "ad-hoc vocabulary namespace for URIs given as string" do
-      # we're wrapping this in a function to isolate the alias
       graph =
-        (fn ->
-           RDF.Graph.build do
-             @prefix ad: "http://example.com/ad-hoc/"
+        RDF.Graph.build do
+          @prefix ad: "http://example.com/ad-hoc/"
 
-             Ad.S |> Ad.p(Ad.O)
-           end
-         end).()
+          Ad.S |> Ad.p(Ad.O)
+        end
 
       assert graph ==
                RDF.graph(
@@ -450,35 +444,41 @@ defmodule RDF.Graph.BuilderTest do
     end
 
     test "two ad-hoc vocabulary namespaces for the same URI in the same context" do
-      # we're wrapping this in a function to isolate the alias
-      graph =
-        (fn ->
-           graph1 =
-             RDF.Graph.build do
-               @prefix ad: "http://example.com/ad-hoc/"
-               @prefix ex1: "http://example.com/ad-hoc/ex1#"
+      graph1 =
+        RDF.Graph.build do
+          @prefix ad: "http://example.com/ad-hoc/"
+          @prefix ex1: "http://example.com/ad-hoc/ex1#"
 
-               Ad.S |> Ad.p(Ex1.O)
-             end
+          Ad.S |> Ad.p(Ex1.O)
+        end
 
-           RDF.Graph.build do
-             @prefix ad: "http://example.com/ad-hoc/"
-             @prefix ex2: "http://example.com/ad-hoc/ex2#"
+      graph2 =
+        RDF.Graph.build do
+          @prefix ad: "http://example.com/ad-hoc/"
+          @prefix ex2: "http://example.com/ad-hoc/ex2#"
 
-             graph1
+          Ad.S |> Ad.p(Ex2.O)
+        end
 
-             Ad.S |> Ad.p(Ex2.O)
-           end
-         end).()
-
-      assert graph ==
+      assert graph1 ==
                RDF.graph(
                  [
                    {
                      RDF.iri("http://example.com/ad-hoc/S"),
                      RDF.iri("http://example.com/ad-hoc/p"),
                      RDF.iri("http://example.com/ad-hoc/ex1#O")
-                   },
+                   }
+                 ],
+                 prefixes:
+                   RDF.default_prefixes(
+                     ad: "http://example.com/ad-hoc/",
+                     ex1: "http://example.com/ad-hoc/ex1#"
+                   )
+               )
+
+      assert graph2 ==
+               RDF.graph(
+                 [
                    {
                      RDF.iri("http://example.com/ad-hoc/S"),
                      RDF.iri("http://example.com/ad-hoc/p"),
@@ -488,22 +488,18 @@ defmodule RDF.Graph.BuilderTest do
                  prefixes:
                    RDF.default_prefixes(
                      ad: "http://example.com/ad-hoc/",
-                     ex1: "http://example.com/ad-hoc/ex1#",
                      ex2: "http://example.com/ad-hoc/ex2#"
                    )
                )
     end
 
     test "merge with prefixes opt" do
-      # we're wrapping this in a function to isolate the alias
       graph =
-        (fn ->
-           RDF.Graph.build prefixes: [custom: EX] do
-             @prefix custom: TestNS.Custom
+        RDF.Graph.build prefixes: [custom: EX] do
+          @prefix custom: TestNS.Custom
 
-             Custom.S |> Custom.p(Custom.O)
-           end
-         end).()
+          Custom.S |> Custom.p(Custom.O)
+        end
 
       assert graph ==
                RDF.graph(TestNS.Custom.S |> TestNS.Custom.p(TestNS.Custom.O),
@@ -516,16 +512,13 @@ defmodule RDF.Graph.BuilderTest do
     test "with vocabulary namespace" do
       import RDF.Sigils
 
-      # we're wrapping this in a function to isolate the alias
       graph =
-        (fn ->
-           RDF.Graph.build do
-             @base TestNS.Custom
+        RDF.Graph.build do
+          @base TestNS.Custom
 
-             ~I<S> |> Custom.p(~I<O>)
-             {~I<foo>, ~I<bar>, ~I<baz>}
-           end
-         end).()
+          ~I<S> |> Custom.p(~I<O>)
+          {~I<foo>, ~I<bar>, ~I<baz>}
+        end
 
       assert graph ==
                RDF.graph(
@@ -546,6 +539,8 @@ defmodule RDF.Graph.BuilderTest do
           {~I<#foo>, ~I<#bar>, ~I<#baz>}
         end
 
+      import RDF.Sigils
+
       assert graph ==
                RDF.graph(
                  [
@@ -566,6 +561,8 @@ defmodule RDF.Graph.BuilderTest do
           {~I<#foo>, ~I<#bar>, ~I<#baz>}
         end
 
+      import RDF.Sigils
+
       assert graph ==
                RDF.graph(
                  [
@@ -585,6 +582,8 @@ defmodule RDF.Graph.BuilderTest do
           ~I<#S> |> EX.p(~I<#O>)
         end
 
+      import RDF.Sigils
+
       assert graph ==
                RDF.graph(~I<http://example.com/base#S> |> EX.p(~I<http://example.com/base#O>),
                  base_iri: "http://example.com/base"